All the superglobal varibales are an associative array and
those varibales are predefined.

1) $_REQUEST
      2) $_GET (URL)
      3) $_POST (form)

4) $_FILES (upload from HTML FORM)
5) $_SESSION (login/logout)
6) $_COOKIE (remember me)
7) $_SERVER (server valribles)
8) $_ENV
==================================
SUPER GLOBAL CONSTANTS

__LINE__
__FILE__
__DIR__
__FUNCTION__
__METHOD__
==================================

<?php

//echo __FILE__;
//echo __LINE__;
//echo __DIR__;
echo "\n\n\n";
function abcs() {
  echo __FUNCTION__;
}
abcs();
echo "\n\n\n";
class A {
  function hello_world() {
    echo __METHOD__;
  }
}
$obj = new A;
$obj->hello_world();

echo "\n\n\n";
